Here's Bieber busting a move to the first chords of his signature song "Baby." Corden's already cracking up.

But that was just a light warm up followed by an utterly crazy hoedown minutes later.

In between their karaoke sessions, Bieber reveals some curious details about his life: Like the fact that he has so many Calvin Klein undies, he never wears the same pair twice — He just throws them away.

Bieber also discussed the romantic tradition of chocolate fondue, yelled "I love you" at some unsuspecting bystanders, texted a girl, and then seamlessly solved a Rubik's Cube.